1. Field of the Invention
The present invention relates to a pseudo halftone process adapted to represent a multi-gradation image with two gradations. In particular, the present invention relates to a quantization process based on a prepared dither pattern.
2. Description of the Related Art
A pseudo halftone process adapted to represent a multi-gradation image with two gradations, i.e., printing (1) and non-printing (2) of a dot, requires a quantization process during converting multivalued density data to binary data. Typical quantization processing methods include an error diffusion method and a dither method, and in particular, the dither method is advantageously used for high-speed printers requiring speed and multicolor printers using a large data quantity in terms of being performable at high speed with less processing load.
A dot array on a print medium is determined by such a quantization process, and in particular, from a highlight part to a low-gradation part, high dispersibility is required in order to visually suppress graininess. Accordingly, in any of the error diffusion method and the dither method, various techniques adapted to increase dot dispersibility in a low-gradation part have been proposed. However, even in the case of employing the quantization process enabling high dispersibility, mutually superimposing multicolor dots on a sheet of paper like color image printing may deteriorate graininess. For example, in the case of employing the completely same quantization process for each of the colors, dots are printed at the same position for all of the colors, increasing graininess. On the other hand, in the case of performing a quantization process for each of the colors in an independent and uncorrelated manner, dispersibility is excellent for each of the colors, but as a result of superimposition, dot sparseness and denseness appear to negatively affect the dispersibility.
In consideration of such a problem, for example, Japanese Patent No. 4574470 discloses a dither pattern producing method that maximizes dispersibility obtained as a result of synthesizing a multiple color dot array. Using a dither pattern described in Japanese Patent No. 4574470 makes it possible to output image quality having high dispersibility and suppressed graininess even in a low-gradation part of a color image while avoiding beading (coagulation of dots).
However, in Japanese Patent No. 457440, a dither pattern for each color is set with a focus on the dispersibility obtained as a result of synthesizing the multiple color dot array, and therefore in the case of printing with a single color, dispersibility may be reduced as compared with a conventional dither pattern produced when only a single color is considered. That is, in the past, it has been difficult to achieve a dither pattern realizing high dispersibility both for a single color and for a mixed color.